1. Field of the Invention
This invention relates generally to wireless systems and, in particular, to determining the location of a wireless transmitting device, and to guiding the search for said wireless transmitting device.
2. Description of the Related Art
Wireless devices include cellular, PCS (Personal Communication Systems), cordless and satellite phones, wireless PDAs (Personal Digital Assistants) and laptop computers, two-way pagers, radio tags, etc. Collectively these are referred to herein as “wireless devices”.
The need for locating, tracking and searching for wireless devices is increasing. Applications include:
Emergency Response:
A notable emergency response service is the enhanced 911 (E911) service mandated by the FCC (Federal Communications Commission) for all cellular licensees, broadband Personal Communications Service (PCS) licensees, and certain Specialized Mobile Radio (SMR) licensees in the United States. E911 service is to provide the operators at the PSAP (Public Safety Answering Point) with information such as calling number, location of serving base station, and caller's Automatic Location Identification (ALI) in longitude and latitude. This automatic information enables the PSAP operators to dispatch emergency response teams for wireless 911 callers in the similar fashion as for the wire line 911 callers. The location accuracy and reliability required by FCC have been revised several times since their first release, compromising what is needed and what is possible. At the time of this invention, the requirements are                For handset-based solutions: 50 meters for 67 percent of calls, 150 meters for 95 percent of calls;        For network-based solutions: 100 meters for 67 percent of calls, 300 meters for 95 percent of calls.        
Pursuit of Criminals:
To allow law enforcement agencies to track and locate wanted criminals who use wireless devices; to allow the public to report information about offensive callers' identification and location.
Tracking of Fraudulent Calls:
Cellular telephone fraud causes huge revenue losses to the service providers. Real-time location of fraudulent calls will help stop the spread use of “cloned” phones.
Recovery of Stolen Vehicles and Valuable Goods:
Wireless devices attached to the protected objects will enable the tracking and recovery thereof.
Target Monitoring and Searching:
Wireless devices attached to the targets can enable the monitoring and searching of such targets, as children, patients, herds, parolees and probationers, controlled materials and equipment, or toxic waste containers.
Depending on the application, wireless location systems face various technological challenges. Achieving high location accuracy anywhere, anytime, under diversified terrain conditions, and at a low cost is a challenge common to many location applications. Particularly for conventional E911 technologies, the prior art solutions are divided into two major categories: network based and handset based. In network based prior art solutions, location accuracy is sensitive to multipath propagation, number of available detection stations, and geographical geometry of the target in relation to the available detection stations. In handset based prior art solutions using GPS, location accuracy is susceptible to blockage in dense urban areas and inside buildings. The handset based prior art solutions also have problems to provide E911 service to legacy devices already in use. To achieve a required accuracy throughout coverage areas and terrains, the cost is often found far beyond the acceptable limit for operators and many end users. In addition, the prior art E911 solutions do not address the need to guide the search for the target on site.